In recent years, semiconductor lasers which directly amplify optical signals have been developed and their use for optical repeaters in long-distance optical transmission paths is contemplated and experiments therefor are considered.
FIG. 1 shows a schematic diagram of the experimental system for optical repeated transmission described in the ECOC '88 Technical Digest (September 1988). In the figure, the light signals from an optical transmitter 12 are passed through optical fibers 13, 16, 19 and 20, and through an optical filter 21, and are then input to an optical receiver 22. The attenuation due to the optical fibers 13, 16, 19 and 20 is compensated for by optical amplifiers 15 and 18. Polarization controllers 14 and 17 are provided in front of semiconductor lasers used as the optical amplifiers 15 and 18. This is because the optical amplifiers have different amplification factor depending on the direction of polarization of input light. A pulse pattern generator 11, a pattern demodulator 23, a clock generator PLL 24, and a bit error-rate detector 25 are also provided for measuring the bit error rate in the experimental system.
In the optical repeater transmission system described above, the polarization controllers disposed in front of the optical amplifiers are controlled by an electronic computer and the configuration of the repeater is therefore complicated and the repeater is therefore costly, and in addition, polarization control requires time on the order of seconds, and cannot follow rapid variations in the polarization state.